The present invention relates to bearing lubrication for an electric machine.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
The term “electric machine” relates in the following description to an electric motor or a generator. The electric machine can be employed as a drive unit for a motor vehicle, with the electric machine capable of forming part of a hybrid drive unit. Typically, an electric machine includes a stator and a rotor. The rotor has a shaft, a rotor pack, and a bearing system, with the bearing system normally having two or more bearings. To protect the interior of the electric machine from contaminants such as dirt, water or oil, use is made of shaft sealing rings.
During operation, the electric machine experiences losses, which heat up the stator and rotor. Such a temperature rise in the stator and rotor is accompanied by a reduction in efficiency. The temperature rise in the bearing system is accompanied by a reduced lifetime due to lower viscosity and carrying capacity of the lubricant.
The increase in motor speed is a trend which represents a special challenge for the bearing system and its lubrication. The optimal solution for high-speed motors is the presence of oil lubrication. This is technically difficult for a side of the electric machine facing away from a transmission since oil has to be conveyed axially through the entire motor and ensured in various driving situations.
A further determining limit condition in the motor vehicle sector is the construction volume for the respective device. In principle, every reduction in the construction volume is important, in order to satisfy the requirements of users. Heretofore, the bearing facing away from the transmission has been lubricated with grease. This requires specific additional axial construction volume for the bearing, in order to provide an adequate amount of grease for the lifetime lubrication. Also, the use of grease has the further drawback that no heat can be transported away from the bearing, and large temperature differences arise between the inner and outer rings. It is not possible to cool the rotor with grease. Likewise, severe limits are set with regard to raising speed in the case of bearings lubricated with grease.
It would therefore be desirable and advantageous to provide an improved electric machine to obviate prior art shortcomings and to have superior bearing lubrication, especially for high speeds, while yet having a small construction volume.